[BUGFIX] Accept alternative notations for setDBinit

Allow quotes and upper-case notation when checking if
$TYPO3_CONF_VARS['SYS']['setDBinit'] is configured properly.

Resolves: #24582
Releases: 6.1, 6.0
Change-Id: I51a5f6cf363077c738656c207648f21d726de72b
Reviewed-on: https://review.typo3.org/19035
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Markus Klein
Reviewed-by: Oliver Hader